Free T4 (Thyroxine) is the primary hormone produced directly by the thyroid gland. While T4 is technically a “pro-hormone” (meaning it is mostly inactive until converted into T3), it is the most abundant thyroid hormone in your circulation.
The “Free” designation is critical: it refers to the tiny fraction of T4 that is not bound to carrier proteins. This unbound hormone is the only version capable of entering your body’s tissues to be converted into the active energy-producing hormone, T3.
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